The invention relates generally to a device for measuring at least one force component during cutting operations on machine tools, and more particularly to the use of such a device for calibrating existing monitoring sensors on machine tools.
In machine tool cutting operations, process forces are set up between the workpiece and tool, resulting in deformation. Between this deformation and the force causing it, there is a correlation. Furthermore, measuring the process forces has assumed special importance in the automation of cutting operations. When the measured process forces deviate outside the usual range in continuous or cyclically repetitive Processes, it can be deduced that the process is no longer proceeding as it should and that correction is needed. However, this deduction is valid only if the measuring accuracy and functional reliability of the sensors employed for monitoring the process forces can be verified from time to time. In other words, the measuring system has to be calibrated with a very precise and reliably functioning measuring instrument.
The prior art force measuring systems measure the forces or partial forces within a machine component or between two machine components. Generally such arrangements cannot be fitted or dismantled without substantial modifications to the machine. A measuring arrangement of this kind is described in Swiss Pat. No. 502,590. It consists of four piezoelectric force measuring elements between two force introduction plates for measuring forces and moments, making possible a clear-cut division of the introduced force into its three spatial components. This measuring arrangement can be placed typically between two moving machine parts, to detect the forces and moments acting at this point. Another measuring arrangement effective in a plane between two machine parts is described in German Patent No. 3,440,670. Here again, four pressure sensitive measuring cells are arranged in a rectangle in an intermediate plate, and at least one shear force component, acting typically on a lathe tool, may be determined by an arithmetic unit. Also known is the provision in a machine part of holes, into which transducers are introduced in the form of measuring pins detecting the material stresses acting longitudinally or transversely to the holes. Such configurations are described for example in German Patent No. 3,407,620 and U.S. Pat. No. 4,412,456. Only partial forces can be determined with these arrangements, moreover they do not allow the detection of defined force components. On the contrary, they constitute monitoring sensors serving mainly for routine monitoring of the machine function.
Another disadvantage of all known measuring arrangements is the need to place them relatively far from the cutting tool. This renders measurement of the actual forces at the tool point difficult or impossible, because the force shunts existing are very hard to control. Also the superimposition of considerable inertial forces must be taken into account. For these reasons, precise measurements of the forces acting at the point of application of the cutting tool, especially their components, has been possible hitherto imperfect. Also, the known multicomponent force measuring systems were not easily adaptable to the particular machine tool and has to be adapted individually to the particular cutting tools, for example lathe tools.
In view of these shortcomings of the known measuring arrangements, the invention is aimed at providing a device for precise measurement of the cutting forces directly on the cutting tool and adaptable to different tools and machine tools.
According to the invention this aim is achieved by integrating a dynamometer in a base module of a modular toolholder, to engage a tool. This base module, with the dynamometer integrated in it, is universally usable. The necessary adaptation to a particular machine tool and tools is effected by appropriately designed interfaces. The base module is combinable preferably with a clamping module having an interface for bracing the tool against the dynamometer, and with a mounting module having an interface for mounting on a carrier on the machine tool side. The clamping and mounting module can therefore be of standard design, for interaction with different standard tools and standardized machine tool carrier systems. In particular, according to a development of the invention, the mounting module may consist of a mounting flange, such as a straight shank or clamping device, fixable on a turret carrier on the machine tool side. The dynamometer (to be understood as an electromechanical transducer arrangement for measuring forces and moments) is placed in the base module of the toolholder so that a tool, such as a lathe tool, may be brought into direct engagement with it. In this way force shunts and measuring errors resulting from these are avoided, assuring the prerequisite for high-precision force measuring. One account of the many possible combinations with different clamping and mounting module, there is moreover the possibility of quantity production of at least the base module of the holder with the dynamometer integrated in it.
Furthermore the high-precision direction of the forces of moments acting on the tool means that tee dynamometer arrangement is suitable for calibrating the monitoring sensors Provided on the machine tool, or for verifying the proper functioning of these. This can be accomplished typically by mounting a dynamometer arrangement according to the invention on the machine tool, and comparing measuring signals yielded by the monitoring sensors of the machine while machining a workpiece with the signals from the dynamometer. In addition, the toolholder with integrated dynamometer is suited for determining significant process parameters in the machining of new materials and/or when using new cutting tool materials, also for direct tool monitoring on larger machines. Moreover the cutting behavior of different machine tools can be compared, because meaningful comparative measurements can be made by virtue of the absence of different force shunt resulting from the machine designs.
The measuring elements of the dynamometer may be sensitive to two or all force components Fx, Fy or Fz. Also, a coolant supply system is integrated in the holder. According to a further development of the invention, the tool is held in the holder so that its cutting plane passes through the centerline of a mounting device provided on the holder or shank. Measuring or evaluation electronics may be integrated in the holder at least in part. The measuring elements are accommodated in the base module preferably so that they are sealed against ingress of coolant and other aggressive media or liquids. A connecting cable for the dynamometer signals may also be integrated in the holder. To enable further toolholders according to the invention to be positioned in the immediate proximity, they should have standard dimensions.
Another aspect of the invention is that it offers a procedure for calibrating the electromechanical monitoring sensors Of machine tools by comparing the signals from a dynamometer arrangement according to the invention fitted on the machine tool with the signals from the monitoring sensors under equal cutting conditions.